Synthesis of structured carbon material from organic materials

ABSTRACT

A method of forming a carbonized composition includes providing an organic composition, forming a protective layer over the organic composition, increasing temperature to carbonize the organic composition and for a period of time to form the carbonized composition, and removing the protective layer from the carbonized composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 62/322,337, filed Apr. 14, 2016, the disclosure of which isincorporated herein by reference.

GOVERNMENTAL INTEREST

This invention was made with government support under grant no.FA9950-13-1-0083 awarded by the Air Force Office of Scientific Research;grant no. CHE-1507629 awarded by the National Science Foundation andgrant nos. N000141310575 and N000141512520 awarded by the Office ofNaval Research. The government has certain rights in this invention.

BACKGROUND

The following information is provided to assist the reader inunderstanding technologies disclosed below and the environment in whichsuch technologies may typically be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technologies or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Graphitic carbon materials (indicating sp2 hybridized carbon materials)and other forms and/or allotropes of carbon have found a wide range ofapplications, such as low-density structures, energy storage structures,and thermal management structures as a result of material propertiesincluding, for example, strength-to-density ratio, porosity, surfacearea, thermal conductivity, electrical conductivity, or a combinationthereof. The internal structure of a carbon material largely defines itsproperties. Therefore, it is desirable to design and synthesizegraphitic/carbon materials with a predetermined structure which may beoptimized for a particular use.

DNA can be readily fabricated into a predetermined, arbitrary-shapedone-dimensional (1D), two-dimensional (2D) or three-dimensional (3D)structures in nanoscale using currently available DNA nanotechnology. Asa template, however, a major limitation of pure DNA nanostructure liesin its limited chemical stability. Hence, almost all reported DNA-basednanofabrications were either based on solution chemistry or conducted atclose to room temperature For example, solution phase metallization onDNA has been demonstrated using various metals (e.g., Ag, Cu, Ni and Au)and can be made site-specific through modification of DNA nanostructurewith binding sites that accept DNA-modified Au or Ag nanoparticles.Vapor phase deposition of metals onto DNA has been used to patternvapor-phase deposited metal. DNA nanostructures may also be used directthe etching and deposition of SiO₂ at room temperature. Although,relatively high quality pattern transfer may be achieved in suchlow-reaction processes, the resultant inorganic nanostructures are oftenof low crystallinity.

High temperature (>500° C.) is often needed for the synthesis andcrystallization of most inorganic materials, such as porous carbon. Thepossibility of using DNA nanostructure to direct chemical synthesis atthis extreme temperature range could create new opportunities inmaterials design and fabrication. However, studies have shown that DNAbegins to degrade at temperatures as low as 130° C. and may completelydegrade at temperatures around 190. It is thus seemingly not possible toachieve pattern transfer from DNA nanostructures under these conditions.

SUMMARY

In one aspect, a method of forming a carbonized composition includesproviding an organic composition, forming a protective layer over theorganic composition, increasing temperature to carbonize the organiccomposition and for a period of time to form the carbonized composition,and removing the protective layer from the carbonized composition. In anumber of embodiments, the organic composition includes a nucleic acid.The organic composition may, for example, consist of a nucleic acid. Thenucleic acids hereof may for example be DNA. The nucleic acid may, forexample, be formed to have a predetermined shape or conformation. Thepredetermined shape or conformation of the nucleic acid may, forexample, be substantially maintained in the carbonized composition. Inthat regard, the height after carbonization (of a nucleic acid or otherorganic composition) may be within 80%, 90%, 95% or even 98% of theheight before carbonization. The nucleic acid or other organiccomposition may, for example, be deposited upon a substrate beforeforming the protective layer over the organic composition.

The carbonized composition may, for example, be a porous carbonmaterial. The temperature may, for example, be increased to atemperature within the range of approximately 780° C. to approximatelythe melting point of the protective layer (2072° C. in the case ofAl₂O₃, for example) or 780° C. to 1000° C. to carbonize the organiccomposition.

The protective layer may be deposited via a thin film depositiontechnique. The protective layer may, for example, be impermeable todecomposition gases of the organic material during carbonization. In anumber of embodiments, the protective layer is deposited via atomiclayer deposition, vacuum deposition, sputtering, chemical vapordeposition or laser assisted deposition. In a number of embodiments, theprotective layer is deposited via atomic layer deposition. The thicknessof the protective layer may, for example, be in the range of 2 nm to 100micrometers. In a number of embodiments, the protective layer comprisesAl₂O₃. The protective layer may, for example, be removed via etchingwith a composition which removes the protective layer withoutsubstantially damaging the carbonized material. The etching compositionmay, for example, comprise H₃PO₄.

In another aspect, a carbonized composition is formed by a processincluding providing an organic composition formed into a predeterminedconfiguration, forming a protective layer over the organic composition,increasing temperature to carbonize the organic composition and form thecarbonized composition, and removing the protective layer from thecarbonized composition, wherein the carbonized composition hassubstantially the predetermined configuration. In a number ofembodiments, the organic composition includes a nucleic acid. In anumber of embodiments, the organic composition consists of a nucleicacid. The nucleic acid may, for example, be DNA.

In a further aspect, a composition includes a nucleic acid; and aprotective layer deposited by atomic layer deposition, vacuumdeposition, sputtering, chemical vapor deposition or laser assisteddeposition over the nucleic acid. In a number of embodiments, theprotective layer is deposited via atomic layer deposition. The thicknessof the protective layer may, for example, be in the range of 2 nm to 100micrometers. The protective layer may, for example, be impermeable todecomposition gases of the organic material during carbonization. In anumber of embodiments, the protective layer comprises Al₂O₃. Theprotective layer may, for example, be removed via etching with acomposition which removes the protective layer without substantiallydamaging the carbonized material. The etching composition may, forexample, comprise H₃PO₄.

In one aspect, a method of forming a carbonized composition includesproviding an organic composition, forming a protective layer over theorganic composition, increasing the temperature to carbonize the organiccomposition and form the carbonized composition, and removing theprotective layer from the carbonized composition. In a number ofembodiments, the organic composition includes or consists of a nucleicacid (for example, DNA). In a number of embodiments, the organiccomposition is formed to have a predetermined shape, structure orconformation (for example, nanostructure). In a number of embodiments,the predetermined shape, structure or conformation (for example,nanostructure) of the organic composition is substantially maintained inthe carbonized composition. The organic composition may be depositedupon a substrate before forming the protective layer thereover.

In another aspect, a carbonized composition is formed by a processincluding providing an organic composition, forming a protective layerover the organic composition, increasing temperature to carbonize theorganic composition and form the carbonized composition, and removingthe protective layer from the carbonized composition.

In a further aspect, a method of forming a carbonized compositionincludes providing a nucleic acid composition and increasing temperatureto carbonize the nucleic acid composition and form the carbonizedcomposition. In a number of embodiments, the method further includesforming a protective layer over the nucleic acid composition beforeincreasing temperature and removing the protective layer from thecarbonized composition. The nucleic acid composition may, for example,include or consist of DNA.

In still a further aspect, a carbonized composition is formed by aprocess including providing a nucleic acid composition and increasingtemperature to carbonize the nucleic acid composition and form thecarbonized composition.

The present devices, systems, and methods, along with the attributes andattendant advantages thereof, will best be appreciated and understood inview of the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates schematically an embodiment of a methodology hereoffor conserving shape or conformation during carbonization ofone-dimensional or 1D DNA structure.

FIG. 1B illustrates an Atomic Force Microscopy (AFM) topographic imageof a 1D DNA structure after deposition on top of Si substrate as setforth in FIG. 1A.

FIG. 1C illustrates an AFM topographic image after atomic layerdeposition (ALD) of Al₂O₃ film as set forth in FIG. 1A.

FIG. 1D illustrates an AFM topographic image after annealing at 800° C.for 5 min as set forth in FIG. 1A.

FIG. 1E illustrates an AFM topographic image after removal of Al₂O₃ filmusing H₃PO₄ as set forth in FIG. 1A.

FIG. 1F illustrates an AFM topographic image after UV/Ozone (UVO)treatment as set forth in FIG. 1A, wherein the AFM images taken on thesame location in FIGS. 1B through 1B.

FIG. 1G illustrates the average height of 1-D DNA after each action orprocess as set forth in FIG. 1A.

FIG. 1H illustrates the height profile of the 1D-DNA structure marked byarrows in FIG. 1D wherein the traces were shifted in the vertical axisfor clarity.

FIG. 1I illustrates the average width of 1-D DNA after each action orprocess as set forth in FIG. 1A, wherein, in FIGS. 1G and 1I, thehorizontal axis represents the five following processes of thefabrication process: (1) after deposition on the Si substrate, (2) afterALD of Al₂O₃ film, (3) after annealing, (4) after removal of Al₂O₃ filmand (5) after UV/Ozone treatment.

FIG. 1J illustrates Raman spectra of 1D-DNA, wherein the AFM heightscale bars for 1D DNA are 10 nm and C-F were AFM images taken on thesame location; in FIGS. 1G and 1I, the horizontal axis represents the 5steps of the fabrication process: (1) after deposition on the Sisubstrate, (2) after ALD of Al₂O₃ film, (3) after annealing, (4) afterremoval of Al₂O₃ film and (5) after UV/Ozone treatment.

FIG. 2A illustrates a map of integrated intensity of the G peak region(1531 to 1661 cm⁻¹), wherein linear features of several micrometers inlength were observed.

FIG. 2B illustrates two representative Raman spectra, one taken from thelinear feature and another from a spot nearby that was Raman-inactive.

FIG. 3A illustrates an AFM topographic image of a DNA triangle afterdeposition on top of Si substrate.

FIG. 3B illustrates an AFM topographic image of a DNA triangle after ALDof Al₂O₃ film.

FIG. 3C illustrates an AFM topographic image of a DNA triangle afterannealing at 800° C.

FIG. 3D illustrates an AFM topographic image of a DNA triangle afterremoval of the Al₂O₃ film, wherein the scale bar for the inset is 200nm.

FIG. 3E illustrates an AFM topographic image of a DNA triangle after UVOtreatment.

FIG. 3F illustrates average height of DNA triangles at each process.

FIG. 3G illustrates average width of DNA triangles at each process.

FIG. 3H illustrates Raman spectra of DNA triangle at each process.

FIG. 4A illustrates an AFM image of the annealed Si/DNA with an Al₂O₃film.

FIG. 4B illustrates Raman spectra of annealed Si/DNA with and without anAl₂O₃ film.

FIG. 4C illustrates an AFM image of the annealed Si/DNA without an Al₂O₃film, wherein; the scale bar for the inset of FIG. 4C is 200 nm, and thecircles in FIG. 4C indicate location of carbon nanostructures.

FIG. 4D illustrates a comparison of the exposed carbon material beforeand after the second annealing.

DETAILED DESCRIPTION

The present devices, systems, methods and compositions, along with theattributes and attendant advantages thereof, will best be appreciatedand understood in view of the following description taken in conjunctionwith any accompanying drawings.

In a number of embodiments, devices, systems, methods and compositionshereof provide for synthesis of carbonized materials/compositions fromorganic materials/compositions such as deoxyribonucleic acid (DNA). Thepresent studies demonstrate that organic materials such as DNA areuseful as material templates for high temperature solid statechemistries.

It will be readily understood that the components of the embodiments, asgenerally described herein and illustrated in the figures hereof, may bearranged and designed in a wide variety of different configurations inaddition to the described example embodiments. Thus, the followingdescription of the example embodiments is not intended to limit thescope of the embodiments, as claimed, but is merely descriptive ofrepresentative embodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, described features, structures, or characteristics may becombined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of embodiments. One skilled in the relevant artwill recognize, however, that the various embodiments can be practicedwithout one or more of the specific details, or with other methods,components, materials, et cetera. In other instances, well knownstructures, materials, or operations are not shown or described indetail to avoid obfuscation.

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “an organic composition”includes a plurality of such organic compositions and equivalentsthereof known to those skilled in the art, and so forth, and referenceto “the organic composition” is a reference to one or more such organiccompositions and equivalents thereof known to those skilled in the art,and so forth. Recitation of ranges of values herein are merely intendedto serve as a shorthand method of referring individually to eachseparate value falling within the range. Unless otherwise indicatedherein, and each separate value as well as intermediate ranges areincorporated into the specification as if it were individually recitedherein. All methods described herein can be performed in any suitableorder unless otherwise indicated herein or otherwise clearlycontraindicated by the text.

In a number of embodiments hereof, one first identifies a candidate forthe organic material/composition, and then converts the organicmaterial/composition to graphitic or carbonized material/compositionwith the shape/conformation of the organic material well conserved. Asused herein, “graphitic carbon” indicates sp2 hybridized carbonmaterial. In a number of embodiments, carbonization of organic materialin the methods hereof resulted in porous carbon nanostructure which wasfound to include approximately 70% graphitic carbon and approximately30% of other forms of carbon when the organic material was carbonized ina temperature range of approximately 780° C. to 1000° C. Carbonizationmay, for example, occur at temperatures above approximately 500° C. In anumber of embodiments hereof, the temperature of carbonization(annealing) was at least 700° C., at least 750° C. or at least 780° C.The upper temperature is limited by the melting point of the protectivelayer.

For the organic material in a number of embodiments, representativestudies hereof included two-dimensional (2D) and three-dimensional (3D)DNA because it can be readily fabricated into a predetermined,arbitrary-shaped structure in nanoscale using currently available DNAnanotechnology. DNA fabrication techniques provide a high degree ofcontrol over size, shape and resolution (with a current theoreticalresolution of approximately 2 nm). After formation of the 2D or 3D DNAstructure, one then converts the DNA structure to a graphitic orcarbonized structure (nanostructure), while preserving or conserving the2D or 3D structure/conformation of the original DNA. Traditionally, DNAapplications can only be conducted at low temperature processes orsolution phase reactions as a result of the limited chemical stabilityof DNA. However, carbonization from DNA to graphitic materials requireshigh temperature in gas phase. Organic materials (for example, polymers)other than DNA may also be used in the formation of the organiccompositions/precursors hereof. Likewise, hybrid materials includingorganic materials other than nucleic acids in combination with nucleicacids (for example, DNA) may be used.

As used herein, the terms “nucleic acid” refers to biopolymers, or largebiomolecules made from monomers known as nucleotides. As used herein,the term “polymer” refers to a chemical compound that is made of aplurality of small molecules or monomers that are arranged in arepeating structure to form a larger molecule. Polymers may occurnaturally or be formed synthetically. The use of the term “polymer”encompasses homopolymers as well as copolymers. The term “copolymer” isused herein to include any polymer having two or more differentmonomers. Copolymers may, for example, include alternating copolymers,periodic copolymers, statistical copolymers, random copolymers, blockcopolymers, graft copolymers etc. As used herein, the term “carbonize”refers to the conversion of a carbon-containing, organic material into acarbon composition at elevated temperature.

To overcome the limitations of the temperature stability of DNA and/orother organic materials, in a number of representative embodimentshereof, the organic material (for example, DNA) structure/layer wasprotected with a thin protective film during the high temperaturereaction and removed the protective film to obtain the graphitic orcarbonized material afterwards. The protective film provides a diffusionbarrier to prevent the organic material/DNA from diffusing into theenvironment, and also maintains the structure/shape of organicmaterial/DNA unchanged during the high temperature reaction in formingthe carbonized material. The material of the protective layers usedherein should be thermally stabile at the high temperatures required tocarbonize the precursor organic materials/structures (for example, 780to 1000° C.). Moreover, the materials of the protective layers should bechemically inert (that is, unreactive with the precursor organiccomposition and with the carbonized composition or product). Theprotective layer may also be removable using a technique that does notsubstantially affect the structure of conformation of the carbonizedcomposition. Examples of suitable materials for use in protective layershereof include, but are not limited to inorganic oxides such as aluminumoxide and silicon oxide. Aluminum oxide may, for example, be removed byH₃PO₄ etching to expose the carbonized composition.

The protective layer may, for example, be formed via a thin filmdeposition technique such as atomic layer deposition. Thin filmdeposition techniques may, for example, be used to apply a thin film ofprotective material having a thickness in the range of approximately fewnanometers to about 100 micrometers, or the thickness of a few atoms.Decreasing film thickness may provide cost savings, but the filmthickness should also be chosen to provide stability of the film inpreventing diffusion of the material and in maintaining the underlyingstructure. Suitable ranges of thickness of the protectively materiallayer are readily determined for a particular material. Protective layerthickness of approximately 20 nm were, for example, found to be suitablefor aluminum oxide (Al₂O₃) in a number of studies hereof. One lowtemperature, chemical vapor deposition (CVD) was found to produce filmswith high porosity, which did not result in high yield carbonization.Atomic layer deposition, vacuum deposition, sputtering, other chemicalvapor deposition techniques and laser assisted deposition may, however,provide dense films which are generally impermeable to gases producedduring carbonization of DNA.

Almost any organic materials can be carbonized using the methodologiesdisclosed herein, including, but not limited to, sugar, syntheticpolymers, cellulose and DNA. Among them, the best substrates forcarbonization include those with aromatic rings. DNA is composed ofthree major components including a phosphate backbone, a sugar, and fourbases. Among the three components, sugar is known to carbonize toproduce amorphous carbon. The four bases are aromatic and structurallysimilar to a large number of compounds (for example, polyimides) thatcarbonize. However, bulk DNA decomposes to produce gaseous products whenheated to temperature above 130° C., making it a challenge to achievepattern transfer from DNA nanostructure at typical carbonizationtemperatures (>500° C.). Although Cu²⁺-impregnated DNA filaments havebeen used to catalyze the growth of graphene nanoribbons, the degree ofshape conservation was not reported. The present studies are the firstto demonstrate precise shape conservation between the DNA templates andthe resulting carbon nanostructures.

Herein, we demonstrate the fabrication of carbon nanostructures throughhigh temperature (ca. 800° C.) shape-conserving carbonization of DNAnanostructures. With a thin Al₂O₃ film coating, a DNA nanostructure canbe converted to carbon nanomaterial while preserving its nanoscaletopography. Porous carbon material plays an important role in a widerange of applications, such aerospace structure, thermal management,fluorescent marker and energy storage.³²⁻⁴⁰ The nanoscale structure ofporous carbon material is essential to its mechanical, thermal, andelectrical properties. For example, nanoscale hierarchical porousstructures can be fabricated to show very high strength (modulus ˜200MPa) at low density (<100 Kg/m³). Currently, the porous carbon materialsare produced by carbonization of organic/polymer precursors in thepresence of an inorganic template. The morphology of existing porouscarbon materials is limited to simple periodic lattices. We also notethat fabrication of 3D, irregular shaped carbon nanostructures isextremely challenging using existing approaches. Because DNAnanostructures (1D, 2D, and 3D) can be made into almost arbitraryshapes, our method has the potential to produce arbitrarily-shaped 1D,2D, and 3D carbon nanostructures.

As illustrated in FIG. 1A, the carbonization procedure includes fourprimary actions or processes. First, a DNA nanostructure was depositedonto a Si wafer substrate. Then, a thin film (for example, having anapproximately 20 nm thickness) of Al₂O₃ was conformally coated onto theDNA nanostructure and the Si substrate by, for example, atomic layerdeposition (ALD). The Al₂O₃-coated DNA nanostructure was then annealedin a low-pressure H₂ atmosphere at high temperature (for example, in therange of 700 to 1000° C. or 800-1000° C.) for a period of time (forexample, 3-5 minutes). Annealing converts the DNA nanostructures tocarbon nanostructures. Finally, the Al₂O₃ coating may be removed by aH₃PO₄ etch to expose the carbon material for further characterizations.Two representative examples are discussed more in detail below.

In a number of studies, a 1D DNA brick crystal was constructed using aDNA brick approach. See, for example, Lee, J.; Kim, J.; Hyeon, T. RecentProgress in the Synthesis of Porous Carbon Materials. Adv. Mater. 2006,18, 2073-2094, the disclosure of which is incorporated herein byreference. An AFM image of the DNA nanostructure is shown in FIG. 1B.The AFM images of FIGS. 1C-1F were taken on the same location. Thestructures are several micrometers in length, 10.1±0.6 nm in height andabout 60±10 nm in width (measured from 10 different samples). Aftercoating the sample with an approximately 20 nm of Al₂O₃ by ALD, the AFMimage of the Al₂O₃ surface still showed the characteristic shape of theDNA nanostructure. The ALD coating process of DNA is thus a conformalcoating process. Therefore, the topography of the DNA is propagated tothe Al₂O₃ surface. We then thermally annealed the Al₂O₃-coated sample at800° C. for 5 min. The highlighted portion of FIG. 1C and FIG. 1D showthe same area of the sample before and after the thermal annealing,respectively. Comparing these two images, there was no change in theshape and relative position of the nanostructures. The melting point ofAl₂O₃ is 2072° C. Thus, the Al₂O₃ provided a stable coating during theannealing process. We then removed the Al₂O₃ coating by a wet etching ofH₃PO₄ to reveal the underlying carbon nanostructures. This etchingtechnique is specific to Al₂O₃ and does not attack carbon or SiO₂. Anytechnique to remove the thin-film, protective layer should notsignificantly affect the carbon nanostructures or the substrate. Theremoval of Al₂O₃ was confirmed by X-ray photoelectron spectroscopy(XPS). After etching with Al₂O₃, the sample was again imaged by AFM atthe same location as illustrated in FIG. 1E. The overall shape orconformation of the nanostructure is observed to be identical to that ofthe DNA template (FIG. 1C). Additional experiments, as further describedbelow, demonstrated that these nanostructures are indeed made of carbon.

To quantify the degree of shape conservations, we measured the averageheight (as illustrated in FIG. 1G) of the nanostructures at each stageof the fabrication. A minor decrease of the height was observed afterALD coating (process 1 to process 2 in FIG. 1A). Without limitation toany mechanism, the reduction in height may arise from shrinkage of theDNA lattice interspace under Al₂O₃ film. Cross sections were measured ona piece of linear DNA crystal, as marked by arrows in FIG. 1D, aftereach processing step. The cross sections showed a high degree ofsimilarity (see FIG. 1H), showing that the shape of DNA nanostructure isconserved from process 2 to process 4 (that is, ALD coating, thermalannealing, and removal of Al₂O₃). Finally, FIG. 1I shows that there wasno change in the width of the nanostructures after carbonization andremoval of Al₂O₃.

Micro-Raman spectroscopy was used to characterize the carbonnanomaterial produced by the carbonization procedure. The DNAnanostructure do not produce detectable Raman signal because of itssmall Raman cross section and low surface coverage. As shown in FIG. 1J,the sample became Raman active after thermal annealing. Both D band(1339 cm⁻¹) and G band (1611 cm⁻¹) peaks were observed. Both peaks arecharacteristic of carbon nanomaterials. The Raman signals persistedafter the removal of Al₂O₃ layer, indicating that the Raman-activematerial was derived from DNA nanostructures underneath the Al₂O₃ film.The presence of G band confirms the formation of sp² hybridization ofcarbon materials (that is, graphitic carbon). The absence of a 2D bandat approximately 2700 cm⁻¹ indicates the lack of a large scale ofconjugated sp² carbon structure. The strong D band indicates thepresence of defects in the DNA-derived carbon material. Withoutlimitation, the D band may originate from several sources: presence ofedges, formation of sp^(a) carbon structure during annealing, andpotentially doping by the heteroatoms in DNA (for example, nitrogen or Natoms). The sp² domain size was estimated to be approximately 8.1 nmaccording to the Tuinstra-Koenig relation. In the Tuinstra-Koenigrelation, I_(D)/I_(G)=C(λ)/L_(a), where the proportionality constantC(λ) depends on the excitation laser wavelength λ. C(λ)=2.4×10⁻¹⁰*λ⁴(for peak-area intensities)=19.2 nm (for λ=532 nm). For 1D DNA crystals,the peak area intensity was 10149 for G peak and 23912 for D peak. Onceagain, the sp² domain size was determined to be L_(a)=8.1 nm. The sp²domain size estimated from the Raman data coincided with the height ofthe carbon nanostructure.

XPS was also used to further confirm the graphitic nature of thenanostructure product. After carbonization and removal of Al₂O₃, the C1speak of the exposed carbon nanostructures shifted to lower bindingenergy from that of the as-deposited DNA. Deconvolution of the C1s peakidentified that the largest contribution in as-deposited DNA sample camefrom the C—H components. After annealing, we observed a significantdecrease in nitrogen content and the sp² C═C species increased from 22%to 70%, confirming that the shape-conserving carbonization producedgraphitize carbon nanostructures.

To further confirm the formation of carbon nanostructures, we subjectedthe annealed sample to an UV/Ozone treatment after the removal of Al₂O₃.Both D and G bands disappeared (see FIG. 1J) after the UV/Ozonetreatment. This observation is consistent with the expected oxidation ofcarbon material by UV/Ozone. Interestingly, the nanostructures werestill visible by AFM and there was no change in their shape and relativeposition (FIG. 1F), although their average height and width decreaseddramatically (FIGS. 1G and 1I). The height profile along an individualDNA crystal structure also showed significant increase of roughness(FIG. 1H). These results indicated that while the carbon materials wereremoved by UV/Ozone treatment, certain oxidation-resistant materialswere left on the surface. Such residues may, for example, be inorganicsalt from the buffer or thermal decomposition products of DNA.

Finally, to confirm that the Raman activity was a result the annealedDNA nanostructure, we carried out confocal Raman mapping of the annealed1D DNA crystal sample over a 10×10 μm area. FIG. 2A shows the map ofintegrated intensity of the G peak region (1531 to 1661 cm⁻¹), wherelinear features of several micrometers in length were observed. FIG. 2Bshows two representative Raman spectra, one taken from the linearfeature and another from a spot nearby that was Raman-inactive. Only thespectrum from the linear structure showed Raman features characteristicof carbon. Those linear structures are consistent with the dimension ofthe DNA nanostructures measured by AFM (FIGS. 1A through 1E), providingdirect evidence that the DNA-to-carbon nanostructure transformation isshape-conserving.

Following the successful shape conserving carbonization of the simplelinear DNA crystal, further studies were made to extend the methodologyto more complex DNA structures. In a number of representative studies,triangle-shaped DNA nanostructures, with a height of 1.6±0.2 nm and awidth of 28.6±5.2 nm on the edge, were selected for their uniquestructural features (for example, linear sides, central void and sharptips) as well as their resistance to aggregation. Unlike the 1D DNAcrystal, which includes 6 overlapping layers of double-stranded DNA, theDNA triangle is made of just one layer of ds-DNA. Considering thecarbonization yield of sugars at 800° C. is only approximately 30%,studies were made to determine if a continuous carbon nanostructure canbe derived from only one layer of ds-DNA.

Similar to the case of 1D DNA crystals, DNA triangles retained theirshape after a series of harsh treatments, including ALD, annealing at800° C. for 5 min and removal of Al₂O₃ by H₃PO₄ etching (see FIG.3A-3D). The average width of the triangle edges changed less than 4%(FIG. 3G), indicating that the DNA nanostructure was well confinedduring the carbonization procedure. The average height decreasedslightly after ALD (from 1.6±0.2 nm to 1.3±0.2 nm) and annealing (to1.1±0.2 nm), but increased (to 1.9±0.2 nm) unexpectedly after theremoval of Al₂O₃ (see FIG. 3F). It is known that the apparent heightmeasured by AFM is sensitive to the tip-substrate interaction and maydeviate from the actual height by as much as 1 nm, especially in caseswhere the sample and the substrate are chemically different (forexample, carbon vs SiO₂). High resolution AFM image was taken after theremoval of Al₂O₃ film. The image presented a continuous, intacttriangular nanostructure with a central void (see FIG. 3D, inset).

Micro-Raman spectroscopy was conducted to detect the presence of carbonmaterials at each step. Similar to the case of 1D DNA crystal, the DNAtriangles sample became Raman active after annealing at hightemperature, showing clear D and G bands. Such Raman features were stillobserved after the removal of Al₂O₃ coating (see FIG. 3H), indicatingsuccessful carbonization of DNA material. After exposure to UV/Ozone,the triangle-shaped nanostructure (FIG. 3E) disappeared along with the Dand G bands in the Raman spectra (FIG. 3H), proving that the triangularnanostructures in FIG. 3D were indeed made of carbon. In a controlexperiment, we also treated the samples with UV/Ozone before removingthe Al₂O₃ coating. In those studies, we observed no change in the Ramanactivity and AFM topography of the sample. These control experimentsshowed that the carbon material was underneath the Al₂O₃ film and thatthe Al₂O₃ coating protects the carbon material from oxidation by O₃.

This shape conserving carbonization approach is compatible with otherDNA templates as well. As another example, we show that a large 2D DNAcrystal, prepared using the DNA brick approach, maintained its shapeafter the carbonization. AFM images of the 2D DNA crystal, before andafter thermal annealing; showed the height profiles present similarsurface features, with the same height of 1.82±0.15 nm. The Ramanspectrum showed that D and G bands appeared after annealing. Inaddition, Raman mapping showed that the Raman activities originate frommicron-sized objects whose dimensions are similar to that of the 2D DNAstructures. This data again demonstrates that the carbonization of DNAnanostructure is a shape-conserving process.

With a thicker 2D DNA crystal of 11.3±0.4 nm in height, we obtainedcarbonized structures with a height of 7.3±0.7 nm after annealing andremoval of Al₂O₃. The electrical properties of this carbon nanostructurewere measured using conductive AFM. With an electrical bias of 2 V, thecurrent measured on the carbon nanostructure is 0.28±0.03 nA, which ishigher than that of the Si substrate. Given that the Si substrate iscovered by a native oxide layer after the removal of Al₂O₃, this resultindicates that the DNA-derived carbon nanostructure is conductive to acertain degree. Although only nA level of current was observed in theconductive AFM measurements, this current may be limited by contactresistance between the AFM tip and the sample. Thus, the intrinsicconductivity of the carbon nanostructure may be much higher.

The inorganic oxide thin film hereof (for example, a 20 nm Al₂O₃ film)is important in the carbonization of organic matter while maintainingthe conformation thereof. In a control experiment, we annealed a DNAtriangle sample without the Al₂O₃ film. Although triangular shapedstructures were still observed after annealing (as illustrated in FIG.4A), they are significantly lower (0.58±0.14 nm) in height, and therewas no D and G band observed (see the lower line of FIG. 4B). It isbelieved that this Raman-inactive structure is the salt residuefollowing decomposition of DNA. In contrast, in the presence of Al₂O₃film even a single layer of ds-DNA is capable of producing carbonmaterial (see the upper line of FIG. 4B) and preserving its nanoscalemorphology. The control experiments hereof also showed that anapproximately 20 nm of Al₂O₃ coating is impermeable to gas at roomtemperature. In a number of embodiments hereof, the thin film ofinorganic oxide is impermeable to gas in which the annealing occursand/or to gases produced from carbonization of the organic material.Without limitation to any mechanism, it is postulated that, in additionto preserving the shape of the nanostructure, the thin-film coating (forexample, a Al₂O₃ coating) also prevents or slows down the decompositionproducts of DNA from escaping. As a result, the thin film increases thecarbonization yield.

With the Al₂O₃ coating removed, the carbon nanostructure broke down tosmall particles after heating at 800° C. for 5 min (see FIG. 4C),indicating poor stability of graphitic structures at high temperature,which may be a result of enhanced diffusion. However, storing anannealed sample (with Al₂O₃ removed) at room temperature did not lead todegradation of the nanostructure, as determined by AFM images and Ramanspectra taken on the same sample. Finally, additional experiments showedthat the carbon nanostructures are stable upon repeated AFM imaging andis not affected by laser induced heating in the time scale of Ramanmeasurements.

In further studies, the annealing conditions were systematically variedto study the effect of temperature, duration and gas environment on thecarbonization. To understand the effect of temperature, we carbonizedthe triangle DNA nanostructure at 780° C., 800° C. and 1000° C. In allthree cases, the Raman spectra showed clear D and G bands, indicatingthat the carbonization occurred over a wide temperature range. Toevaluate the effect of annealing time, two Al₂O₃-coated DNA trianglesamples were annealed at 800° C. for 5 min and 20 min, respectively.Raman spectra and AFM images showed that in both cases,shape/conformation conserving carbonization occurred. Additionally, thesame 1D DNA crystal sample was annealed at 800° C. for 5 min and thensubjected to 1000° C. annealing for another 3 min. AFM topography imagesand height profiles indicated remarkable preservation of nanostructure.Also, Raman spectra showed D and G peaks after both annealing processes.These results demonstrate that the carbonization was completed within 5minutes, and the carbon structure can be preserved at high temperatureduring extended heat treatment, owing to the high melting point of Al₂O₃film. The effect of gas environment was studied by heating theAl₂O₃/DNA/Si samples in H₂, Ar and air at 800° C. for 5 min. In the caseof H₂ and Ar, the Raman spectra show apparent D and G peaks, while nographitic signal was observed from the samples annealed in air.Furthermore, for the sample annealed in H₂, we subject it to a secondannealing in air at 800° C. for another 5 min, and the D and G peaksvanished. Compared with the previous demonstration of additionalannealing in H₂ atmosphere, the results indicate that the carbonizationprocedure should be carried out in inert atmosphere since the Al₂O₃ filmwas not impermeable to O₂ at high temperature, although it does provideprotection against UV/O₃ oxidation.

Carbon source other than DNA nanostructure could be introduced duringthe carbonization process. Possible non-DNA carbon source include theairborne carbon contamination, the byproduct of ALD, and the buffersolution used for DNA deposition. Control experiments were conducted todetermine the possible contribution from all these sources, as describedbelow.

A Raman spectrum taken from annealed Al₂O₃/Si, which was prepared fromdirect deposition of Al₂O₃ on a blank Si surface (without DNA), showedno D or G peak, indicating the ALD product residue and airborne carboncontaminations do not produce carbon material. Similarly, we found thatthe buffer solution did not introduce a significant amount of carbonprecursor. During the deposition of the 1D DNA crystals, the Si waferwas rinsed with water after the DNA deposition. A control sample wasprepared by soaking a Si wafer in a DNA-free buffer solution, followedby rinsing the wafer with water. This sample was then coated with Al₂O₃.No carbon material was detected by Raman spectroscopy after thermalannealing. The preparation for DNA triangle and 2D DNA crystal samplesinvolve rinsing with an ethanol-water mixture. In this case, a controlsample (Si wafer soaked in DNA-buffer, then rinsed with ethanol-watermixture) showed weak D and G peaks in the Raman spectrum after ALDcoating and annealing. However, the Raman signal intensity was only22%-27% of that from samples having deposited 1D and 2D DNA crystals.

Experimental

Preparation of DNA Nanostructure on Si Substrate

Preparation of DNA nanostructure: Synthetic and M13mp18 DNA forpreparing the DNA triangle origami were purchased from IDT and NewEngland Biolabs, respectively. The 2D DNA triangles was formed byheating the DNA solution to 95° C. followed by a slow cooling to 25° C.in 24 hrs. The resulting solution was purified by centrifuging 6 timesto remove the extra short strands. DNA solution was made from TAE/Mgbuffer solution (12.5 mM Mg(OAc)₂, 40 mM Tris, 20 mM acetic acid and 2mM EDTA). 1D-DNA crystals and 2D DNA crystals were prepared using theDNA brick approach. The buffer solution contains 40 mM Mg²⁺.

Deposition of DNA nanostructure on Si wafer: Silicon wafers werepurchased from University Wafers. It was cleaned with hot piranhasolution (7:3 (v/v) of concentrated H₂SO₄: 35% H₂O₂). Triangular DNAorigami was assembled on the substrate by dripping 2 μL of DNA solutionon the substrate and waiting for 40 min before blow away the solution.The substrate was immersed in a 9/1 (v/v) ethanol/water solution toremove the salt from the buffer solution. 1D-DNA was assembled bydripping 2 μL of DNA solution for 4 min and then washing with 400deionized water. After the deposition of DNA, the substrate was precededto the deposition step within one day.

Deposition of Protective Inorganic Film

Atomic layer deposition (ALD) of Al₂O₃ on DNA/Si substrate: We usedtrimethylaluminum as precursor. The chamber and substrate heaters wereset to 200° C. and the Throttle valve position was set to give 200 mtorrat 260 sccm total Ar flow. The deposition looped 200 times of 0.006 sTMA pulse, 10 s interval, 0.06 s H₂O pulse and 10 s interval. Thepre-set deposition thickness of both oxide films was 20 nm and theexperimental thickness of the film was measured by ellipsometry. Thesurface of the sample was imaged using tapping mode AFM. Surprisingly,the DNA nanostructures survived the relatively harsh conditions of ALDwhile maintaining their original conformation.

Annealing Experiment

Typically, the prepared sandwich-like substrate was placed at the centerof quartz plate in a 1-inch-diameter fused quartz tube. The furnace tubewas evacuated and H₂ gas flowed at speed of 2.0 standard cubiccentimeters per minute (sccm) with a pressure of 70 mTorr for 5 min.Then the furnace was heated to 800° C. under a 2.0 sccm of H₂. Time wasrecorded when the temperature reach the setting value. Then thesubstrate was cooled to room temperature under H₂ gas flow and taken outfrom the tube furnace.

Etching Experiment

The Al₂O₃ film was etched in the 4.56 M H₃PO₄ solution for 1 hour,followed by rinsing with 1 M H₃PO₄ and H₂O. The etching procedure wasstudied on the annealed Al₂O₃/SiO₂ wafer and the etching rate is about0.3 nm/sec.

UV-Ozone Experiment

The substrate was subjected in the NOVASCAN® PSD Pro Series UV-Ozonecleaner for UVO treatment (available from Novascan Technologies, Inc. ofAmes, Iowa). The UV/O3 chamber was flushed with oxygen for 5 min beforeUV irradiation. The typical duration for the treatment was 60 min.

Characterization Methods

Raman spectroscopy: Typically, the Raman spectra were measured using anANDOR iDus® Raman microscope (available from Andor Technology LTD ofBelfast, North Ireland) equipped with solid state 532 nm laser (2.33 eV)with a spot size of ˜1 μm (through a 40× lens). Each Raman trace wastaken with 20 to 600 seconds integration time under a low incident laserpower of 1.2-1.4 mW, thus the heating effects can be neglected.

Confocal Raman mapping: The confocal Raman mapping was performed usingRenishaw inVia Raman microscope, with 633 nm laser excitation. Thespatial step was 0.5 μm, and the integration time for each spot was 10sec. The laser power was 1.7 mW and the grating was 18001/mm.

Atomic force microscopy: Surface morphology was measured using tappingmode atomic force microscopy (AFM) using a VEECO® Dimension 3100scanning probe microscope (available from VEECO Instruments Inc.) or anASYLUM RESEARCH® MFP-3D atomic force microscope (available from OxfordInstruments Research, Inc. of Santa Barbara, Calif.) with μMasch® NSC15AFM tips (available from MikroMasch USA of Watson, Calif.) in air.Contact mode and conductive AFM images were taken on an ASYLUM RESEARCH®MFP-3D AFM using an ORCA™ module (available from Oxford InstrumentsResearch, Inc.) and BUIDGETSENSORS® Tap300E-G AFM tips (available fromInnovative Solutions Bulgaria, Ltd. Of Sofia, Bulgaria) in air.

Ellipsometry: Thickness measurements of the oxide film were carried outon an alpha-SE® Ellipsometer. The literature refractive index value ofSiO₂, Al₂O₃ was 1.450 and 1.921 respectively. The refractive index wasalso measured by using Cauchy self-fitting model.

X-Ray Photoelectron Spectroscopy (XPS): XPS was conducted in theESCALAB™ 250XI XPS microscope (available from Thermo Fisher Scientificof Cleveland, Ohio). Deconvolution of the C1s peak was calculated usingXPSPEAK 4.1 software (freeware). We note that the carbon XPS data shouldbe interpreted with caution because airborne hydrocarbon couldcontaminate the surface. This contamination is known to occur on SiO₂surface; we recently also reported that the contamination occurs ongraphitic surface as well.

The foregoing description and accompanying drawings set forth a numberof representative embodiments at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope hereof, which is indicated by thefollowing claims rather than by the foregoing description. All changesand variations that fall within the meaning and range of equivalency ofthe claims are to be embraced within their scope.

What is claimed is:
 1. A method of forming a carbonized composition,comprising: providing an organic composition comprising a nucleic acid;forming a protective layer over the organic composition; increasingtemperature to carbonize the organic composition and for a period oftime to form the carbonized composition; and removing the protectivelayer from the carbonized composition.
 2. The method of claim 1 whereinthe organic composition consists of a nucleic acid.
 3. The method ofclaim 1 wherein the nucleic acid is DNA.
 4. The method of claim 1wherein the nucleic acid is formed to have a predetermined shape orconformation.
 5. The method of claim 4 wherein the predetermined shapeor conformation of the nucleic acid is substantially maintained in thecarbonized composition.
 6. The method of claim 5 wherein the nucleicacid is deposited upon a substrate before forming the protective layerover the organic composition.
 7. The method of claim 1 wherein thecarbonized composition is a porous carbon material.
 8. The method ofclaim 1 wherein the temperature is increased to a temperature within therange of approximately 780° C. to approximately 2072° C.
 9. The methodof claim 1 wherein the protective layer is deposited via a thin filmdeposition technique.
 10. The method of claim 9 wherein the protectivelayer is deposited via atomic layer deposition, vacuum deposition,sputtering, chemical vapor deposition or laser assisted deposition. 11.The method of claim 10 wherein the protective layer is deposited viaatomic layer deposition.
 12. The method of claim 9 wherein the thicknessof the protective layer is in the range of 2 nm to 100 micrometers. 13.The method of claim 1 wherein the protective layer comprises Al₂O₃. 14.The method of claim 13 wherein the protective layer is removed viaetching with H₃PO₄.
 15. The method of claim 1 wherein the protectivelayer is impermeable to decomposition gases of the organic compositionduring carbonization.